Method for producing piezoelectric thin-film resonator

ABSTRACT

A method for producing a piezoelectric thin-film resonator includes forming a sacrificial layer on a substrate, performing a plasma treatment on the sacrificial layer so that the surface roughness (Ra) of end surface portions of the sacrificial layer is about 5 nm or less, forming a strip-shaped dielectric film so as to be continuously disposed on the surface of the substrate and the end surface portions and the principal surface of the sacrificial layer, forming a piezoelectric thin-film area including a lower electrode, an upper electrode, and a piezoelectric thin-film disposed therebetween so that a portion of the lower electrode and a portion of the upper electrode surface each other at an area on the dielectric film, the area being disposed on the upper portion of the sacrificial layer, and removing the sacrificial layer to form an air-gap between the substrate and the dielectric film.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a piezoelectric thin-film resonator anda method for producing the same.

2. Description of the Related Art

In piezoelectric thin-film resonators, a vibrating portion in which apiezoelectric thin-film is disposed between a pair of excitationelectrodes facing each other must be acoustically separated from asubstrate. For this purpose, the vibrating portion (membrane) must bepartially lifted from the substrate with an air-gap layer therebetween.

In a piezoelectric thin-film resonator having this type of structure, avibrating portion includes a supporting portion supported on a substrateand a lifting portion lifted from the substrate. The lifting portion issupported by the supporting portion, and thus a stress is easilyconcentrated in the vicinity of the boundary between the lifting portionand the supporting portion.

Japanese Examined Patent Application Publication No. 6-40611 discloses apiezoelectric thin-film resonator having such a structure. Thepiezoelectric thin-film resonator described here is often used as aterminal part for mobile communication or a part for BLUETOOTH™communication devices. In recent years, devices themselves have beensignificantly reduced in size because the communication frequency hasbecome higher. The frequency is determined by the following equation:Resonant frequency=frequency constant of the piezoelectric diaphragm/T,wherein T represents the thickness of the vibrating portion.

The frequency constant of the piezoelectric diaphragm is a constantdetermined by the piezoelectric material. For example, when apiezoelectric thin-film resonator is used in the 2 GHz band, thethickness of a piezoelectric film is about 2 μm, the thickness of eachelectrode is about 0.1 μm, and the thickness of an air-gap layer isabout 1 μm.

As described above, the thickness of the air-gap layer has become verysmall and thus it is known that the finish of the electrodes provided onthis air-gap layer having a small thickness significantly affects thecharacteristics of the piezoelectric thin-film resonator.

As described above, piezoelectric thin-film resonators have beensignificantly reduced in size as the communication frequency has becomehigher. Consequently, new problems, which have not been considered asimportant problems in the past, have arisen. The problems will bedescribed below.

An air-gap layer is formed according to the following process. First, asacrificial layer is formed on a substrate. A main portion of apiezoelectric thin-film resonator is formed on the surface of thesacrificial layer so as to be continuous with the surface of thesubstrate. Subsequently, the sacrificial layer is removed by etching orthe like to form the air-gap layer.

When a crystalline material such as zinc oxide is used for thesacrificial layer, the smoothness of end surface portions of thesacrificial layer is generally deteriorated compared with the principalsurface of the sacrificial layer. The surface roughness (Ra) of the endsurface portions of the sacrificial layer is about 10 nm to about 20 nm.As described above, as the frequency becomes higher, the thickness ofthe sacrificial layer becomes very small. Consequently, thedeterioration of the smoothness of the end surface portions causes thefollowing problems:

1) The smoothness of a dielectric film provided on the sacrificial layeris also deteriorated. Furthermore, crystallinity of a lower electrodeprovided on the dielectric film is deteriorated, resulting in theincrease in wiring resistance. Thereby, resonance characteristics aredeteriorated.

2) It becomes difficult to cover the sacrificial layer with a thindielectric film. On the other hand, when the thickness of the dielectricfilm increases in order to improve the covering ability, resonancecharacteristics are deteriorated.

3) If the covering by the dielectric film is insufficient, an area isformed where the sacrificial layer is in contact with the lowerelectrode. In such a case, when the sacrificial layer is removed, thelower electrode is corroded by an acid or the like used for removing thesacrificial layer. The corrosion of the electrode deteriorates resonancecharacteristics.

These problems become more obvious because as the communicationfrequency becomes higher, devices are reduced in size, and accordingly,the thickness of the air-gap layer becomes very small.

SUMMARY OF THE INVENTION

In order to overcome the problems described above, preferred embodimentsof the present invention provide a piezoelectric thin-film resonatorhaving improved resonance characteristics and electric power resistance.

A preferred embodiment of the present invention provides a method forproducing a piezoelectric thin-film resonator including the steps offorming a sacrificial layer on a substrate, performing a plasmatreatment on the sacrificial layer so that the surface roughness (Ra) ofend surface portions of the sacrificial layer is about 5 nm or less,forming a strip-shaped dielectric film so as to be continuously disposedon the surface of the substrate and the end surface portions and theprincipal surface of the sacrificial layer, forming a piezoelectricthin-film area including a lower electrode, an upper electrode, and apiezoelectric thin-film disposed therebetween so that a portion of thelower electrode and a portion of the upper electrode face each other atan area on the dielectric film, the area being disposed on the upperportion of the sacrificial layer, and removing the sacrificial layer toform an air-gap between the substrate and the dielectric film.

According to a preferred embodiment of the method for producing apiezoelectric thin-film resonator of the present invention, thesacrificial layer may include a crystalline compound.

According to a preferred embodiment of the method for producing apiezoelectric thin-film resonator of the present invention, thesacrificial layer may include zinc oxide.

According to a preferred embodiment of the method for producing apiezoelectric thin-film resonator of the present invention, thesacrificial layer may include an organic polymer.

According to a preferred embodiment of the method for producing apiezoelectric thin-film resonator of the present invention, thesacrificial layer may include Ge, Sb, Ti, Al, or Cu.

According to a preferred embodiment of the method for producing apiezoelectric thin-film resonator of the present invention, the plasmatreatment may be a method in which RF discharge is performed with aninert gas such as Ar or He and the sacrificial layer is sputter-etchedby the self bias.

According to a preferred embodiment of the method for producing apiezoelectric thin-film resonator of the present invention, a gascontaining an element provided in the sacrificial layer may be used as agas for the plasma treatment.

According to a preferred embodiment of the method for producing apiezoelectric thin-film resonator of the present invention, an oxide maybe used for the sacrificial layer and oxygen may be used as the gas forthe plasma treatment.

A preferred embodiment of the present invention provides a piezoelectricthin-film resonator including a substrate on which a sacrificial layeris provided, a strip-shaped dielectric film that is continuouslydisposed on the surface of the substrate and end surface portions andthe principal surface of the sacrificial layer, and a piezoelectricthin-film area including a lower electrode, an upper electrode, and apiezoelectric thin-film disposed therebetween wherein a portion of thelower electrode and a portion of the upper electrode face each other atan area on the dielectric film, the area being disposed on the upperportion of the sacrificial layer, and an air-gap between the substrateand the dielectric film, the air-gap being formed by removing thesacrificial layer. In the piezoelectric thin-film resonator, a plasmatreatment is applied so that the surface roughness (Ra) of the endsurface portions of the sacrificial layer is about 5 nm or less.

According to preferred embodiments of the present invention, since endsurface portions of a sacrificial layer for forming an air-gap areplanarized, the covering ability of a dielectric thin-film provided onthe sacrificial layer can be improved. As a result, an electrodeprovided on the dielectric thin-film is not simultaneously etched duringthe etching of the sacrificial layer. Therefore, an electrode materialthat is corroded by a dilute acid can be used for the electrode, andthus the device itself can be inexpensively produced. In addition, sincecorrosion of the electrode by a dilute acid can be prevented, defects ofthe electrode are not generated. Thus, a satisfactory resonator can beproduced.

As in the operation described above, corrosion of a piezoelectricmaterial by the dilute acid can also be prevented, and thus asatisfactory resonator can be produced.

According to preferred embodiments of the present invention, since endsurface portions of the sacrificial layer for forming the air-gap areplanarized, the surface smoothness of the dielectric thin-film providedon the sacrificial layer can also be improved and crystallinity of theelectrode provided on the dielectric thin-film can be improved.Consequently, the wiring resistance can be decreased to produce asatisfactory resonator. Similarly, a resonator having excellent electricpower resistance can be produced.

According to preferred embodiments of the present invention, theplanarization of end surface portions of the sacrificial layer forforming the air-gap can simultaneously planarize the principal surfaceof the sacrificial layer, resulting in the improvement of theorientation of a piezoelectric film. Thus, a resonator havingsatisfactory characteristics can be produced.

Other features, elements, steps, characteristics and advantages of thepresent invention will become more apparent from the following detaileddescription of preferred embodiments of the present invention withreference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1H are views showing central cross-sections of apiezoelectric thin-film resonator according to a preferred embodiment ofthe present invention in the order of production.

FIG. 2 is a perspective view of the piezoelectric thin-film resonatoraccording to a preferred embodiment of the present invention in which asubstrate disposed at the bottom is not shown.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

A sacrificial layer for forming an air-gap layer is provided on asubstrate. A dielectric film, a lower electrode, a piezoelectric film,and an upper electrode are provided on the sacrificial layer. Such apiezoelectric resonator is produced as follows.

FIGS. 1A to 1H are cross-sectional views showing the order of productionof a piezoelectric thin-film resonator according to a preferredembodiment of the present invention. FIG. 2 shows a perspective view ofa final drawing of the piezoelectric thin-film resonator according to apreferred embodiment of the present invention. A method for producingthe piezoelectric thin-film resonator according to a preferredembodiment of the present invention will now be described step-by-stepwith reference to FIGS. 1A to 1H.

FIG. 1A shows an example of a substrate 1 serving as a bottom layer of apiezoelectric resonator. A substrate that is inexpensive and that hasexcellent workability is used as the substrate 1. A silicon or glasssubstrate having a smooth surface is suitable for the substrate 1.

FIG. 1B shows an example of an intermediate product in which asacrificial layer 2 is provided on the substrate 1. The sacrificiallayer 2 for forming an air-gap layer 7 is provided on the substrate 1 bytechniques such as sputtering and photo etching, for example. Thematerial of the sacrificial layer 2 should withstand a high temperaturewhich the intermediate product may reach when a piezoelectric thin-film5 is formed, and should be easy to chemically remove. Typically, zincoxide is suitable. Other examples of the material include metals such asGe, Sb, Ti, Al, and Cu; phosphosilicate glass (PSG); and organicpolymers. Preferred examples of the organic polymers includepolytetrafluoroethylene and derivatives thereof, polyphenylene sulfide,polyether ether ketone, polyamides, polyamide-imides, polyimides,polyimide-siloxane, vinyl ethers, polyphenyl, parylene-n, parylene-f,and benzocyclobutene. With respect to the thickness of the sacrificiallayer 2, it is necessary that when a membrane serving as a vibratingportion 8 is bent, the vibrating portion 8 is not in contact with thesubstrate 1. In view of ease of the formation, the thickness of thesacrificial layer 2 is preferably about 0.5 μm to about severalmicrometers. The minimum distance between end surface portions of thesacrificial layer 2 and the vibrating portion 8 is about 50 times thethickness of the vibrating portion 8 or less. Preferably, the endsurface portions of the sacrificial layer 2 provide a substantiallytapered shape and the angle formed between each of the end surfaceportions and the substrate is about 15 degrees so that a stress appliedon bending portions of electrodes can be reduced.

FIG. 1C shows a step of a plasma treatment of the sacrificial layer 2.The sacrificial layer 2 provided in the previous step is planarized by aplasma treatment. The plasma treatment may be either ion etching orplasma etching, for example. Ion etching includes the followingprocesses: CCP-RIE (Reactive Ion Etch), Dual-frequency CCP-RIE, ECREtch, Helicon-wave Plasma Etch, and Ion Milling. In the case of usingion for planarization, RF discharge is performed with an inert gas suchas Ar or He, and sputter etching may be performed by the self bias. Thatis, when the substrate 1 is positioned over a ground electrode, DCelectrode, or an AC electrode so that the substrate 1 will go throughion etching, an electric potential lower than the plasma is provided tothe substrate 1. This electric potential attracts ions within the plasmato bring about the collision by ions with the substrate 1. Sputteretching includes etching an object by use of the physical energy due tothis collision. According to sputter etching, an electric field isconcentrated on protruded portions, that is, ions are concentrated onprotruded portions so that physical collision energy is intensivelyprovided on protruded portions and the sacrificial layer 2 can beeffectively planarized. Even in sputter etching, when radicals reactivewith the object are generated in the plasma, the following problems mayoccur. For example, the radicals react with the surface of thesacrificial layer to form a layer that is difficult to etch or, on thecontrary, the sacrificial layer is etched. In this regard, however, themethod of using plasma of an inert gas (rare gas) such as Ar or He isadvantageous because such radicals are hardly generated. Otherwise,planarization can be achieved without using ions by positioning thesubstrate at float potential, which is so-called plasma etching, forexample. In this case, no bias is applied on the substrate and collisionof the ions on the substrate is difficult to occur. Therefore, in plasmaetching, chemical reactions between neutral radicals in the plasma andthe object are mainly utilized. In this case, however, the reactionproceeds while irregularities formed before planarization are maintainedas they are. Therefore, the sacrificial layer is sometimes planarizedless effectively than the case by sputter etching.

When the sacrificial layer 2 includes an oxide such as zinc oxide,gaseous oxygen may be used. In the case where the sacrificial layerincludes ZnO, preferably the surface of the sacrificial layer is pureZnO. The ZnO layer is finally removed by etching. In this final etchingstep, if the surface of the sacrificial layer is not in the pure stateof ZnO, for example, if a layer including a compound containing anothersubstance has been formed on the surface or if the bonding state of ZnOhas been changed, the etching may not easily start or, in the worstcase, etching may not be performed, resulting in the failure offormation of an air-gap. When only the inside of the sacrificial layercan be etched but the outermost layer thereof cannot be etched, eventhough the air-gap can be formed, the remaining outermost layer servesas a leak path or the like, resulting in deterioration of electricalproperties of the device and a decrease in the ratio of non-defectiveproducts. However, when oxygen, which is an element of ZnO, is used asplasma, a compound containing an element other than ZnO, i.e., acompound that is difficult to etch, is not formed on the surface of thesacrificial layer. Accordingly, the plasma treatment does notdestabilize the step of etching the sacrificial layer. In this way, inplace of the above-described inert gas, the plasma treatment may beperformed with a gas, such as gaseous oxygen, which is an element of thesacrificial layer. This can also prevent problems from occurring. Thesurface roughness (Ra) of the end surface portions of the sacrificiallayer is preferably about 5 nm or less.

FIG. 1D shows an example of a dielectric film 3 that continuously coversthe surface of the substrate 1 and the surface of the sacrificial layer2. The dielectric film 3 is preferably formed by sputtering, chemicalvapor deposition (CVD), electron beam evaporation, or the like so as tocover the entire surface of the sacrificial layer 2. A nitride such asaluminum nitride, which is an insulating material and has satisfactorythermal conductivity, or silicon nitride, which has excellentpassivation property, is preferably used for the dielectric film 3.Furthermore, the material used for the dielectric film 3 preferably hasfrequency temperature characteristics opposite to those of the materialused for the piezoelectric thin-film 5. In such a case, the change infrequency relative to the change in temperature of the resonator filteris reduced to improve the characteristics. When zinc oxide or aluminumnitride is used for the piezoelectric thin-film 5, silicon oxide, whichhas frequency temperature characteristics opposite to those of thesematerials, is preferably used.

FIG. 1E shows an example of an intermediate product in which a lowerelectrode 4 is formed on the dielectric film 3 formed in the previousstep. The lower electrode 4 is formed on the dielectric film 3 by a stepof forming the film by sputtering, plating, CVD, electron beamevaporation, or the like and a step of patterning using aphotolithography technique. In these steps, the lower electrode 4 mainlyincluding a metallic material such as Mo, Pt, Al, Au, Cu, or Ti iscontinuously formed on the sacrificial layer 2 and the substrate 1 so asto have a strip shape. The surface roughness of the lower electrode 4 ispreferably about 2 nm or less.

FIG. 1F shows an example of a state in which a piezoelectric thin-film 5is provided on the dielectric film 3 including the lower electrode 4.The piezoelectric thin-film 5 including zinc oxide, aluminum nitride, orthe like, is provided on the dielectric film 3 having the lowerelectrode 4 thereon by a step of forming the film by sputtering or thelike and a step of patterning using a photolithography technique. Whenan aluminum nitride film is provided, the patterning is performed by wetetching with an alkaline solution.

FIG. 1G shows an example of an intermediate product in which an upperelectrode 6 is provided on the piezoelectric thin-film 5 formed in theprevious step. The upper electrode 6 is formed on the piezoelectricthin-film 5 by the same method as that in the lower electrode 4. Inorder to improve the overall strength, a second dielectric film isfurther formed thereon in some cases.

FIG. 1H shows an example of a state in which the sacrificial layer 2 isremoved to form the air-gap layer 7. A photoresist film is patterned byphotolithography to form an etch hole used for removing the sacrificiallayer 2. This photoresist film also functions as a protective film forprotecting the upper electrode 6. After the etch hole is formed, aportion of the dielectric film 3 on the sacrificial layer 2 is removedby reactive ion etching, wet etching, or the like. For example, whensilicon oxide is used for the dielectric film 3, a reactive ion etchingis performed using a fluorine-containing gas such as CF₄. Alternatively,a wet etching may be performed with a solution such as hydrofluoricacid. After the etching, the etch mask such as the photoresist isremoved with an organic solvent such as acetone. Alternatively, a dryetching using an oxygen etching may be performed.

Finally, the sacrificial layer 2 is etched to form the air-gap layer 7.A photoresist or the like is patterned by photolithography and thesacrificial layer 2 is then removed by reactive ion etching, wetetching, or the like. For example, when zinc oxide is used for thesacrificial layer 2, the zinc oxide is removed with an acidic solutionsuch as hydrochloric acid or phosphoric acid. After the etching, theetch mask such as the photoresist is removed with an organic solventsuch as acetone. When a solution that does not etch any of thepiezoelectric thin-film 5, the dielectric film 3, the lower electrode 4,and the upper electrode 6 is used, the steps of patterning byphotolithography and removing the etch mask can be eliminated. Forexample, when zinc oxide is used for the sacrificial layer 2, aluminumnitride is used for the piezoelectric thin-film 5, silicon oxide is usedfor the dielectric film 3, and Pt, Au, Ti, or the like is used for thelower electrode 4 and the upper electrode 6, the sacrificial layer 2 canbe easily removed with, for example, a mixed solution of acetic acid andphosphoric acid without patterning. After the etching, the etchant andthe like are sufficiently replaced by a volatile liquid such asisopropyl alcohol and drying is then performed. Thus, the air-gap layer7 is formed.

While preferred embodiments of the present invention have been describedabove, it is to be understood that variations and modifications will beapparent to those skilled in the art without departing the scope andspirit of the invention. The scope of the present invention, therefore,is to be determined solely by the following claims.

What is claimed is:
 1. A method for producing a piezoelectric thin-filmresonator comprising the steps of: forming on a substrate a sacrificiallayer having a top principal surface and end surface portions;performing a plasma treatment on the sacrificial layer so as toplanarize the sacrificial layer and reduce a surface roughness of theend surface portions and the top principal surface of the sacrificiallayer to 5 nm or less; forming a strip-shaped dielectric film so as tobe continuously disposed on a surface of the substrate, the end surfaceportions, and the top principal surface of the sacrificial layer;forming a lower electrode on the dielectric film; forming apiezoelectric thin-film on the dielectric film on which the lowerelectrode has been formed; forming an upper electrode on thepiezoelectric thin-film such that a portion of the upper electrode and aportion of the lower electrode face each other on an upper surface ofthe sacrificial layer with the piezoelectric thin-film disposedtherebetween; and removing the sacrificial layer to form an air-gapbetween the substrate and the dielectric film.
 2. The method forproducing a piezoelectric thin-film resonator according to claim 1,wherein the sacrificial layer comprises a crystalline compound.
 3. Themethod for producing a piezoelectric thin-film resonator according toclaim 2, wherein the sacrificial layer comprises zinc oxide.
 4. Themethod for producing a piezoelectric thin-film resonator according toclaim 1, wherein the sacrificial layer comprises an organic polymer. 5.The method for producing a piezoelectric thin-film resonator accordingto claim 1, wherein the sacrificial layer comprises Ge, Sb, Ti, Al, orCu.
 6. The method for producing a piezoelectric thin-film resonatoraccording to claim 1, wherein the plasma treatment includes performingan RF discharge with an inert gas and the sacrificial layer issputter-etched by the self bias.
 7. The method for producing apiezoelectric thin-film resonator according to claim 1, wherein a gascontaining an element of the sacrificial layer is used as a gas for theplasma treatment.
 8. The method for producing a piezoelectric thin-filmresonator according to claim 7, wherein an oxide is used for thesacrificial layer and oxygen is used as the gas for the plasmatreatment.
 9. The method for producing a piezoelectric thin-filmresonator according to claim 1, wherein the end surface portions of thesacrificial layer have a substantially tapered shape forming an angle ofabout 15 degrees with respect to the surface of the substrate.